Is Diphenhydramine Bad for You? Risks Explained

Diphenhydramine, the active ingredient in Benadryl and dozens of over-the-counter sleep aids, carries a surprisingly long list of risks for a drug you can buy without a prescription. At standard doses it impairs driving more than alcohol does, and at higher or chronic doses it has been linked to increased dementia risk, cardiac complications, and a withdrawal syndrome that catches regular users off guard. A growing number of allergy and pharmacology experts now argue the drug has outlived its usefulness entirely, given that safer alternatives exist for every condition it treats.

What Diphenhydramine Does Inside Your Brain

Diphenhydramine was designed to block histamine, the chemical your body releases during an allergic reaction. The problem is that it does not stop there. A PET imaging study found that a standard oral dose occupies roughly 56% of the brain’s H1 histamine receptors, which explains the heavy sedation most people feel after taking it.¹ But histamine receptors are only part of the story. Diphenhydramine also blocks muscarinic acetylcholine receptors with potency comparable to some dedicated anticholinergic drugs.² That anticholinergic action is responsible for the dry mouth, blurred vision, urinary retention, and constipation people experience. It also affects the heart: diphenhydramine competitively inhibits the muscarinic receptors that help regulate heart rate, which matters at higher doses or in people with pre-existing cardiac conditions.³

In short, diphenhydramine is a blunt pharmacological instrument. It blocks multiple receptor systems at once, and the side effects people experience are not bugs in the design but direct consequences of that broad receptor activity.

Driving and Short-Term Impairment

One of the most underappreciated risks of diphenhydramine is how severely it impairs your ability to do everyday tasks, especially driving. In a randomized trial using a driving simulator, participants who took diphenhydramine performed worse behind the wheel than participants who were legally drunk.⁴ That is not a subtle finding. Many people take diphenhydramine for allergies or a cold and then drive to work without a second thought, yet the evidence suggests they would be safer driving after a drink.

Beyond driving, diphenhydramine impairs psychomotor performance more broadly and makes people feel substantially worse. A separate study found that a single 100 mg dose impaired coordination and severely worsened how participants rated their own mental state.⁵ Adding alcohol or sedatives on top of diphenhydramine compounds these effects further, a combination people stumble into regularly since both diphenhydramine and alcohol are so freely available.

The Sleep Aid That Undermines Sleep

Diphenhydramine is the active ingredient in most over-the-counter sleep aids sold in the United States, including ZzzQuil, Unisom SleepGels, and many store-brand versions. People reach for it because it reliably makes them drowsy. But making you drowsy and giving you good sleep are two different things.

In animal studies, low doses of diphenhydramine increased non-REM sleep while reducing REM sleep. At higher doses the picture actually reversed: non-REM sleep decreased, the brain’s slow-wave patterns were disrupted, and wakefulness increased.⁶ REM sleep is essential for memory consolidation and emotional regulation, so a drug that suppresses it is arguably making your sleep worse even if you feel like you are sleeping longer.

There is also the tolerance problem. In a controlled study, diphenhydramine caused clear sedation on day one, but by day four the sleepiness it produced was indistinguishable from a placebo.⁷ A separate trial confirmed the same timeline: impairment from diphenhydramine was gone by the third day of use, apparently because tolerance developed that quickly.⁸ This means people who take it nightly as a sleep aid are likely getting no real sedative benefit within a few days, while still accumulating the anticholinergic side effects. The dry mouth and grogginess the next morning persist even after the sleep benefit has faded.

The Dementia Question

The most alarming body of evidence around diphenhydramine involves long-term cognitive risk. Because diphenhydramine is a “strong anticholinergic,” it falls into a class of drugs that has been repeatedly linked to higher rates of dementia in observational studies. The research does not prove causation, but the pattern is consistent enough that it has changed prescribing guidelines.

The landmark study followed over 3,400 older adults for an average of about seven years. Those with the highest cumulative anticholinergic exposure had roughly 54% higher risk of developing dementia and 63% higher risk of Alzheimer’s disease specifically, compared with people who used none of these drugs. The relationship was dose-dependent, meaning the more total anticholinergic medication someone had taken over the prior decade, the higher their risk.⁹

A more recent study focused specifically on first-generation antihistamines like diphenhydramine in patients with allergic rhinitis. The pattern was similar: cumulative use showed a dose-dependent increase in dementia risk, with the highest-use group facing about 51% greater risk than non-users.¹⁰ The dose-response curve is what makes these findings harder to dismiss as coincidence: a little anticholinergic exposure is associated with a small increase in risk, and a lot of exposure with a larger increase.

Brain imaging studies add a structural dimension to these findings. Anticholinergic users showed worse performance on cognitive tests, reduced brain glucose metabolism, and larger ventricles, a marker of brain atrophy.¹¹ A separate study of community-dwelling older adults found that midlife anticholinergic use was associated with faster rates of cortical gray matter loss in brain regions important for memory and attention.¹²

A fair caveat: all of these studies are observational. It is possible that people who use more anticholinergic drugs have underlying health conditions that independently raise their dementia risk. But the consistency across studies, the dose-response relationship, the supporting brain-imaging data, and the known role of acetylcholine in memory all point in the same direction. Most geriatricians now actively avoid prescribing diphenhydramine to older patients, and some professional organizations list it as a drug that should be avoided in people over 65.

Why Older Adults Are Especially Vulnerable

Aging bodies process diphenhydramine more slowly, and aging brains are more sensitive to its anticholinergic effects. A study of hospitalized patients aged 70 and older found that those given diphenhydramine had 70% higher risk of developing delirium symptoms compared with unexposed patients. The risk of specific delirium features was even starker: inattention tripled, altered consciousness tripled, and disorganized speech increased more than fivefold.¹³

Delirium in an older hospitalized patient is not a minor inconvenience. It extends hospital stays, increases the risk of falls, and is associated with higher mortality. The fact that an over-the-counter antihistamine can trigger it highlights how poorly matched diphenhydramine is for this population. If you have an older parent or grandparent who keeps Benadryl in the medicine cabinet for allergies or sleep, this is worth a conversation.

Overdose and Cardiac Toxicity

At therapeutic doses, diphenhydramine’s cardiac effects are usually minor. At high doses, the picture changes. Diphenhydramine blocks fast sodium channels in the heart, and at still higher concentrations it interferes with potassium channels responsible for cardiac repolarization. The result is a prolonged QT interval, a measurable change in heart rhythm that raises the risk of dangerous arrhythmias.¹⁴ Case reports of massive ingestions describe widened QRS complexes, wide complex tachycardia, and QT prolongation on presentation.¹⁵

These are not theoretical risks. Emergency departments see diphenhydramine overdoses regularly, both intentional and accidental. The drug’s wide availability and reputation as a mild sleep aid lead some people to take far more than the labeled dose, assuming that “more is better” or that an over-the-counter product cannot be truly dangerous. That assumption is wrong. Large overdoses of diphenhydramine can be fatal, primarily through cardiac arrest or seizures.

Misuse, Abuse, and the “Benadryl Challenge”

Diphenhydramine has a well-documented history of intentional misuse. At very high doses it produces hallucinations, delirium, and psychotic symptoms, effects driven by its anticholinergic activity rather than any traditional “high.” Case reports of diphenhydramine-induced toxic psychosis have been appearing in emergency medicine literature for decades.¹⁶ More recently, social media challenges have encouraged teenagers to take large quantities of diphenhydramine for hallucinogenic effects, resulting in hospitalizations and at least a handful of reported deaths.

A systematic review of over-the-counter drug misuse found that people who abuse diphenhydramine report seeking relaxation, calmness, and improved sleep. Acute intoxication, however, is associated with psychotic symptoms, agitation, restlessness, and disorientation. Chronic misuse can produce physical dependence, with withdrawal symptoms including bowel and bladder incontinence, high blood pressure, muscle rigidity, anxiety, irritability, and intense rebound insomnia.¹⁷ The hallucinations produced by antihistamine abuse are generally described as deeply unpleasant and frightening rather than euphoric, which makes the pattern of continued misuse all the more striking.¹⁸

What Happens When You Stop Taking It

People who have been taking diphenhydramine regularly, whether for allergies, sleep, or anxiety, sometimes find that stopping abruptly is harder than they expected. Abruptly discontinuing anticholinergic drugs can trigger a cholinergic rebound syndrome, with symptoms like nausea, sweating, and urinary urgency. Stopping histamine-blocking drugs specifically can produce activation, insomnia, and a mild withdrawal syndrome.¹⁹

The rebound insomnia is particularly tricky because it convinces people that they truly “need” diphenhydramine to sleep. In reality, the difficulty sleeping after stopping is a temporary withdrawal effect, not evidence that the drug was working well. Tapering off gradually, rather than stopping cold, can reduce these symptoms for people who have been using it nightly for weeks or months.

Drug Interactions Worth Knowing About

Diphenhydramine is metabolized by liver enzymes and can interfere with the metabolism of other drugs processed through the same pathways. In particular, it inhibits CYP2D6, a liver enzyme responsible for breaking down a wide range of medications including certain antidepressants, beta-blockers, and pain medications.²⁰ If you are taking a drug that depends on CYP2D6 for clearance, adding diphenhydramine can effectively increase the other drug’s blood levels, potentially amplifying its side effects.

The sedative interaction is even more straightforward and more dangerous. Combining diphenhydramine with alcohol, benzodiazepines, opioids, or other central nervous system depressants multiplies the sedation and respiratory depression from each individual substance. Because diphenhydramine is so freely available, people often do not think to mention it when asked about their medications, and clinicians do not always ask about it.

Why Second-Generation Antihistamines Replaced It

Drugs like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) were specifically developed to treat allergies without the sedation and anticholinergic side effects of older drugs like diphenhydramine.²¹ These newer antihistamines largely stay out of the brain, which is why they do not make most people drowsy and do not carry the same cognitive risks. For the vast majority of allergy sufferers, there is simply no reason to choose diphenhydramine over a second-generation alternative.

For sleep, the situation is more complicated because diphenhydramine’s drowsiness was its selling point. But given the rapid tolerance, poor sleep architecture effects, anticholinergic burden, and dementia concerns, most sleep medicine specialists now recommend against it for anything beyond a rare, one-off night. Cognitive behavioral therapy for insomnia remains the first-line treatment for chronic sleep problems, with medications like melatonin receptor agonists or low-dose doxepin as pharmaceutical options that carry less baggage.

Growing Calls to Restrict Over-the-Counter Access

Diphenhydramine is available in more than 300 formulations, the vast majority sold without a prescription. A recent expert commentary in a major allergy journal argued that diphenhydramine has reached the end of its useful life and represents a greater public health hazard than other drugs in its class. The authors recommended that it should no longer be widely prescribed or remain so easily available over the counter.²² That recommendation has not yet translated into regulatory action, but it reflects a growing consensus among allergists and pharmacologists that the drug’s risk-benefit ratio no longer makes sense in an era of safer alternatives.

Part of the challenge is cultural. Diphenhydramine has been around since the 1940s. Multiple generations grew up with Benadryl in the medicine cabinet and think of it as fundamentally harmless. That perception lags behind the science by decades.

Diphenhydramine in the Water Supply

A less obvious consequence of diphenhydramine’s popularity is its environmental footprint. The drug has been detected in surface water, sediment, and fish tissue, though comprehensive understanding of its effects on aquatic ecosystems remains limited.²³ Research on the Niagara River found diphenhydramine among the major pharmaceutical pollutants accumulating in various fish organs, alongside antidepressants and their metabolites.²⁴ Wastewater treatment does not fully remove pharmaceutical compounds, and the sheer volume of diphenhydramine consumed globally means it enters waterways continuously. What that bioaccumulation means for fish behavior, reproduction, or the broader food web is still an open question, but the fact that the drug is accumulating in wildlife tissue at all adds another dimension to the argument that its overuse has consequences beyond any individual pill bottle.